This invention relates to an improved process for preparing a white halogenated bisimide product having good filterability and a low Yellowness Index (Y.I.) as measured by ASTM 1313.
As is taught in U.S. Pat. No. 4,374,220, there are a multitude of halogenated bisimides which are effective as flame retardants in formulation with macromolecular flammable materials, e.g. polymers. These formulations are useful in making articles such as wire insulation and electronic housings. Of the halogenated bisimides, the N,N'-alkylene-bis(tetrabromophthalimide)s and the N,N'-bis(tetrabromophthalimide)s are especially commercially significant.
A presently used commercial route for producing a product which principally contains N,N'-alkylene-bis(tetrabromophthalimide) comprises reacting tetrabromophthalic anhydride with a diaminoalkane in the presence of water and an alkanoic acid to yield a reaction mass containing the intermediate, N,N'-alkylene diammonium-bis(tetrabromophthalate). The reaction mass is then heated to about 225.degree. C. for a period of about 2 hours to convert the intermediate to N,N'-alkylene-bis(tetrabromophthalimide) which is the principal constituent of the product recovered from the reaction mass. This product is particularly useful as it has good thermal stability and resistance to UV degradation. However, the product has a yellow color which argues against its presence in compositions used for forming white articles. Also, the intensity of the yellow color can vary between product batches, which color variance, if severe, can make it difficult for the article manufacturer to maintain consistency in the color of the articles produced. The yellow color is believed to be due to impurities formed during the conversion of the N,N'-alkylene diammonium-bis(tetrabromophthalate) intermediate to the corresponding bisimide product.